To reduce manufacturing costs it is desirable to cocast plural layer films instead of coating one or more layers on a previously cast film. The prior art discloses apparatus for the simultaneous deposit of two or more layers of solutions or dispersions. For example, the patent to Bartlett et al, U.S. Pat. No. 2,932,855, discloses a multislotted die for simultaneously casting a plurality of layers of polymer compositions on a moving drum. Prior art devices, however, are not satisfactory for casting two or more layers with a high degree of uniformity when the viscosities of the casting solutions or dispersions (also known as dopes in non-aqueous systems) or the thicknesses of the layers differ greatly. Films of this kind include, photographic films that have a magnetic recording layer as disclosed, for example, in the patent to Krall, U.S. Pat. No. 3,782,947. They have a thin magnetic layer and a much thicker cellulosic support layer. Also, they are coated from dopes which differ greatly in viscosity and cannot be cocast precisely with conventional casting dies.
The present invention provides a novel apparatus and method by means of which plural layer films can be cocast with high precision despite great differences in the thicknesses of the layers and the viscosities of the dopes. For example, dope viscosity ratios can be as high as 300,000 to 1 and thickness ratios can range from 5000:1 to 1:1.